1. Technical Field
The present invention relates generally to a device, method, and system for retrieving fracturing balls used in the process of hydraulic fracturing. More particularly, the present invention relates to a fracturing ball retrieval device, method and system.
2. Description of Related Art
The process of hydraulic fracturing can be used to stimulate a well to maximize the extraction of natural gas or oil. This process can utilize the injection of high-pressure fluids and proppant into the wellbore to create and hold open fractures in the rock. In some processes, a horizontal wellbore is utilized. The wellbore can be drilled thousands of feet deep vertically and thousands more feet horizontally. The horizontal section can be completed and fractured in sections from the toe of the wellbore (or the far end of the horizontal run) back to the heel of the wellbore (where the wellbore becomes vertical and starts to rise towards the surface). Two methods for completing and fracturing the horizontal wellbore are known as “plug and perf” and “sliding sleeve.” Both methods can utilize frac balls, and in some cases, these frac balls need, or are desired, to be removed.
In the “plug and perf” method, once the wellbore is drilled, a wireline crew can be utilized for the purpose of lowering tools and explosives by means of a crane and large winch truck or wireline unit with thousands of feet of cable. The wireline crew can send down a tool string with a perforating gun to fire off and create perforations in the toe section of the wellbore. The wireline crew can then remove the gun and inspect it to make sure it fired properly to create holes in the wellbore casing and cracks in the formation in that section of the well.
If the guns fired properly, then a fracturing crew can be utilized for the purpose of handling fluids and controlling pressure in the wellbore. The fracturing crew can pump proppant into the wellbore to fill and expand (fracture) the cracks formed by the perforating gun.
The wireline crew can then send down a tool string with a perforating gun and a plug on the end. The wireline crew can set the plug just outside the first fractured section. The plug can be designed to allow fluid and pressure to pass through until a frac ball is pumped down and seated in the plug to isolate the section. This fluid and pressure pass-through feature can be important because the hydraulic use of fluid and pressure can be critical to moving things through the wellbore and in forming the wellbore, and isolating a section removes that space from fluid movement and pressure operations.
On the same trip after setting the plug, the wireline crew can fire the perforating guns to fracture the subsequent section. The guns can then be pulled out and inspected to insure proper firing. If the guns fired properly, then a frac ball can be pumped down to seat inside the plug and isolate the previous section. This process can be repeated for each subsequent section. When the entire wellbore is formed, the balls and plugs can be drilled out to start production.
In an attempt to save time and fluid usage, the frac ball can be placed inside the plug prior to running the tool string with the perforating guns and the plug down the wellbore. The plug can then be set with the frac ball already seated, which is known as “ball-in-place,” and the guns can then be fired on the same run. However, this can be considered risky and is often not done because isolating a section can prevent fluid and pressure movement through the isolated section. If ball-in-place is used and the guns don't fire properly, a hydraulic lockout can occur. The proper firing of the guns can be necessary to create perforations in the section of rock to prevent lockout after isolating a section. These perforations can create space for fluid and pressure to move into, which can be necessary to maintain movement of fluid and tools in the wellbore after a previous section is isolated. If these perforations are not formed, it can be desired or necessary to regain access to the isolated section. However, the seated frac balls can prevent access.
When a wellbore is hydraulically locked, costly and time-consuming measures often must be taken to regain fluid movement capabilities, such as electrical tractor conveyance methods and rigid tubing conveyance methods. Another costly and time-consuming measure that can be used is flowback, whereby surface lines can be opened to allow fluid to return to the surface from downhole pressure. Flowback volumes and rates can be limited by equipment and safety requirements and the possibility of natural gas, hydrogen sulfide, petroleum or chemicals returning to the surface can require substantial safety precautions. Using flow back to return seated frac balls from a plug to the surface can sometimes be difficult as a high rate of flow for an extended period of time can be required, or sometimes not practically obtainable. Wellbore sand or other materials may be too heavy or difficult to return to the surface. Flowback can also be used to pull a frac ball out of a plug partially, but it can re-seat itself and can cause lockout again. As such, flowback operations can require days and thousands of barrels of fluid to get enough access to the zone to continue operations. Flowback can also require substantial disposal requirements.
Gun misfires are common enough that it is often considered not feasible to take this risk. Therefore, ball-in-place runs are often not done and guns are pulled out and inspected before pumping down and seating a frac ball to isolate a section. Current methods for removing the frac ball can be considered inadequate to compensate for the risk of making ball-in-place runs because they can require a lot of time, special and expensive equipment, fluid usage, safety issues, and thus great expense.
Even when a gun fires properly and a ball is pumped down and set to isolate a section, lockouts can also occur with certain formation difficulties in the rock that prevent proper injection of fluid. In such a case, it can also be necessary or desired to remove a seated frac ball from an isolated section to regain hydraulic control of the wellbore.
Another way a lockout can occur is with “screen-outs” that can be caused by the wellbore becoming clogged by proppant and sand or can be caused otherwise where the formation at the perforations are not capable of accommodating additional fluid injection. The fracturing crew may have to utilize flowback to clear out the wellbore or remove some of the proppant in formation. In such a case, it can also be necessary or desired to remove a seated frac ball from an isolated section to regain hydraulic control of the wellbore.
The “sliding sleeve” method also possess the risk of screen out or improper sliding and opening of the sleeves, and could also benefit from the effective and efficient removal of fracturing balls.